This application relates generally to disc drive data storage devices and more particularly to an apparatus and method of writing servo track and timing information thereon.
Disc drives are the most common means of storing electronic information in use today. Ordinary disc drives are typically constructed with the following internal components: one or more magnetic media discs attached to a spindle; a spindle motor that rotates the spindle and the attached discs at a constant high speed; an actuator assembly, located adjacent to the discs, with a plurality of actuator arms that extend over the discs, each with one or more flexures extending from the end of each actuator arm, and with a read/write head mounted at the distal end of each flexure; and a servo positioner that rotates the actuator assembly about a bearing shaft assembly positioned adjacent to the discs such that the read/write heads radially traverse the disc surface (i.e., move back and forth the between the inner and outer diameters of the disc).
Information is stored on and retrieved from a magnetizable material on the disc""s surface. To facilitate information storage and retrieval, discs are radially divided in concentric circles known as xe2x80x9cservo tracksxe2x80x9d or xe2x80x9ctracksxe2x80x9d. The tracks are assigned a track number so that the servo positioner can locate a specific track. The servo positioner, upon receiving a control command, aligns the read/write head over the desired track. The process of switching between different tracks is called xe2x80x9cseekingxe2x80x9d, whereas remaining over a single track while information is stored or retrieved is called xe2x80x9cfollowingxe2x80x9d.
Each track is subdivided into pie-shaped sections, called xe2x80x9csegmentsxe2x80x9d or xe2x80x9csectorsxe2x80x9d. There may be ten to a hundred, or even more, sectors dispersed around a single track. Each sector usually has one or more servo sectors (also known as servo marks among others) associated with it. A servo sector contains information that is used by the servo positioner to determine the radial position of the head relative to the disc surface and relative to the track center. Servo sectors typically consist of a Gray code field, which provides coarse position information such as the track and cylinder number, and a servo burst field, which provides fine position information such as the relative position of the read/write head to the track center. Generally speaking, the servo burst field creates a signal with a specific voltage magnitude when the read element is not aligned over the track centerline. The signal is decoded to pinpoint the read element""s location and the read element is moved directly over the centerline by positioning the read element such that the sum of the servo burst field voltages equals zero.
Whereas servo sectors are used to determine the radial position of the read/write head relative to the disc surface, the servo positioner uses a timing signal mechanism to determine the circumferential position of the read/write head relative to the disc surface. A timing, or clock, burst (also commonly referred to as a timing or clock, mark, pulse, or signal among others) consisting of a series of magnetic transitions of a constant frequency is used as a timing signal mechanism by most servo positioners. In a typical disc drive, timing bursts are written at various circumferential locations around each track. Usually each segment of the track contains a timing burst. For simplicity, the magnetic transitions of the timing burst, depending on their phase, will be referred to as xe2x80x9cevenxe2x80x9d or xe2x80x9coddxe2x80x9d transitions.
The servo positioner can use the timing bursts in conjunction with an index signal to determine the circumferential location of the read/write head relative to the disc surface. The index signal, usually obtained from a spindle motor driver, goes active at a predetermined spindle motor position in each disc revolution. In most disc drives, the spindle motor rotates the discs at a constant velocity and the circumferential location of the read/write head relative to the disc surface can easily be determined by measuring the time interval between the index signal and a timing burst.
Tracks, servo sectors, and timing bursts are usually written on the disc during the manufacturing process, using one of two means: 1) a servowriting machine, or 2) self-propagated servo writing. A servowriting machine is a large piece of external equipment that uses a very accurate lead screw and laser displacement measurement feedback device to precisely align a write element. The write element, which is attached to an external head/arm positioner, is aligned relative to where the desired track is to be written on the disc surface. A track is written on the disc once the write element is correctly aligned. An external clock signal is then used to write the servo sectors and timing pulses onto the track. The head/arm positioner then moves the write element a predetermined distance to the next desired track location. The next track, along with corresponding servo sectors and timing bursts, is written. The process repeats until the desired number of tracks is written onto the disc.
A servowriter, however, has several drawbacks. First, a typical disc may contain more than 60,000 servo tracks. The process of aligning and writing each track with its corresponding servo sectors and timing bursts is very time consuming and expensive. Second, although very accurate at lower track densities, the servowriter cannot meet the accuracy requirements dictated at higher track densities. Finally, the servowriter procedure must be completed in a clean room because the disc components are exposed during servowriting; again adding expense to the servowriting procedure.
The second means of writing tracks on a disc is called self-propagating servo writing. Oliver et al. first described this method of servo track writing in U.S. Pat. No. 4,414,589. Several other patents have disclosed slight variations in the Oliver patent, but the same basic approach is used. Under the basic method, the drive""s actuator arm is positioned at one of its travel range limit stops. A first reference track is written with the write element. The first reference track is then read with the read element as the read/write head is radially displaced from the first reference track. When a distance is reached such that the read element senses a predetermined percentage of the first reference track""s amplitude, a second reference track is written. The predetermined percentage is called the xe2x80x9creduction numberxe2x80x9d.
For example, the read element senses 100% of the first reference track""s amplitude when the read element is directly over the first reference track. If the reduction number is 40%, the read/write head is radially displaced from the first reference track until the read element senses only 40% of the first reference track""s amplitude. A second reference pattern is written to the disc once the 40% is sensed by the read element. The read/write head is then displaced in the same direction until the read element senses 40% of the second reference track""s amplitude. A third reference track is then written and the process continues. The process ends when the actuator arm""s second limit stop is reached and the entire disc surface is filled with reference tracks. The average track density is then calculated using the number of tracks written and the length of travel of the read/write head.
If the average track density is too high, the disc is erased, the reduction number is lowered so that a larger displacement occurs between tracks, and the process is repeated. If the track density is too low, the disc is erased, the reduction number is increased so that a smaller displacement occurs between tracks, and the process is repeated. If the track density is within the desired range, the reduction number for the desired average track density has been determined, the disc is erased, and servo tracks are written to the disc by alternatively writing servo and reference tracks. Each track is then further divided by writing servo sectors and timing bursts around the track""s circumference.
Although Oliver et al. did not disclose a method of propagating timing marks within servo tracks, Chanier et al. (U.S. Pat. Nos. 5,581,420, 5,901,003, and 5,757,574) disclosed several methods for propagating timing marks. The methods disclosed by Chanier et al., however, are not applicable to disc drives in which the radial offset, between the read/write head""s read element and write element, is larger than the width of the write element and/or the width of the read element.
A new method of self-propagating servo track writing called Extended Copying with Head Offset, or ECHO, introduced by Seagate Technology LLC uses a radial offset between the read and write elements, which is greater than the width of the write element and/or the width of the read element to write servo tracks on the disc surface. The ECHO process utilizes the offset between the read and write element such that as the write element is writing a new servo track, the read element is gathering positional information from a previously written servo track. In other words, the write element""s position over the desired track location is maintained as the read element follows a previously written track. The radial offset, between the read element and the write element, is several tracks wide in some embodiments of the ECHO process. Therefore, Chanier""s methods for propagating timing bursts cannot be used with ECHO process.
Accordingly there is a need for a method of writing timing bursts on disc drives that can be used for disc drives in which the offset between the read element and the write element is greater than the width of the write element.
Against this backdrop an embodiment of the present invention has been developed. The embodiment proposes a timing burst writing technique for use with conventional servowriting and self-propagating servo track writing, including the ECHO process. In one embodiment, a servo track guide pattern containing servo sectors and timing bursts are written onto the surface of a disc using a conventional servowriter. The timing information consists of a series of xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d bursts. By convention, the first, third, fifth, etc. timing bursts following a servo sector are referred to as xe2x80x9codd burstsxe2x80x9d, while the second, fourth, sixth, etc. timing bursts following a servo sector are referred to as xe2x80x9ceven bursts.xe2x80x9d Each burst consists of a series of magnetic transitions. The write element is aligned over the servo track to have timing bursts written. The read element, offset from the write element, remains aligned over the servo track guide pattern. The phased-lock-loop (xe2x80x9cPLLxe2x80x9d) controller is synchronized to the even timing bursts when the read element traverses an even timing burst. When the write element traverses an area where an odd burst is to be written, the controller enables the write element and a series of magnetic transitions constituting an odd burst is propagated (i.e., written) onto the disc surface. After the odd burst is written to the track, the PLL is synchronized to the odd burst as the read element is activated. When the write element traverses an area where an even burst is to be written, the write element is enabled and a series of magnetic transition constituting an even burst is propagated onto the disc surface. When all the desired timing bursts are written on a track, the head is displaced until the write element is aligned over the next track to have timing bursts written. The process of synchronization, writing, and displacing is repeated until all of the desired tracks contain timing bursts.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.